1. Field of the Invention
This invention relates to an automatic gain control circuit or the like, and more particularly to a variable attenuator for use in such a high frequency band as a microwave band.
2. Description of the Related Art
Conventionally, variable attenuators have a .pi.-shaped circuit form or a bridged T-shaped circuit form, and both employ PIN diodes at terminations of resistors so that the attenuation amount may be varied by varying the resistance values of the PIN diodes, that is, by varying the voltages applied to the PIN diodes.
Concrete examples of a conventional variable attenuator are described below.
(1) Variable attenuator disclosed in Japanese Patent Laid-Open Application No. Heisei 3-155210.
In the conventional example disclosed, an inverse logarithmic amplifier is used as a control circuit for controlling PIN diodes and in addition the attenuation characteristic with respect to a voltage outputted from the inverse logarithmic amplifier is offset to correct the non-linearity of the attenuation characteristic.
(2) Variable attenuator disclosed in Japanese Patent Laid-Open Application No. Showa 57-44314.
In the conventional example disclosed, a quarter wavelength line is used, and the quarter wavelength line and PIN diodes are connected in parallel to each other so that the attenuation amount is varied by one control current while maintaining a fixed resistance.
However, such conventional variable attenuators as described above have the following problems.
FIG. 1(a) is a circuit diagram showing a radio frequency circuit of the variable attenuator disclosed in Japanese Patent Laid-Open Application No. Heisei 3-155210, and FIG. 1(b) is a schematic view showing an example of mounting of the circuit shown in FIG. 1(a).
As shown in FIGS. 1(a) and 1(b), the variable attenuator disclosed in Japanese Patent Laid-Open Application No. Heisei 3-155210 includes a pair of resistors 15 and 16 connected in series between PIN diodes 13 and 14 and the ground, respectively.
FIG. 2 is a circuit diagram showing a radio frequency circuit of the variable attenuator disclosed in Japanese Patent Laid-Open Application No. Showa 57-44314.
As shown in FIG. 2, the radio frequency circuit of the variable attenuator disclosed in Japanese Patent Laid-Open Application No. Showa 57-44314 includes a pair of quarter wavelength lines 133 and 134, a pair of terminal resistors 115 and 116, and a pair of capacitors 126 and 127 for removing dc components.
Since the conventional variable attenuators have such constructions as described above, they have a problem in that a shield case for accommodating the high frequency circuit has a large size and the isolation is degraded by an unnecessary resonance or propagation mode.
Further, where they are used in a microwave band of a frequency higher than 10 GHz, since ideal parts such as resistors and capacitors are not available, they have a distributed constant characteristic depending upon the parasitic capacitances or the shapes and there is the possibility that the attenuator characteristic may be degraded.
Further, where resistors or like elements are printed on a circuit board using a thin film or thick film technology, there is an additional problem that an increase in cost is invited.